Methane Oxidation over, and Sulfur Interactions with, Pd/Pt Bimetallic Catalysts

Using Pt/Pd/Al2O3 catalysts, the effect of Pt/Pd ratio, and H2O and SO2 exposure on complete CH4 oxidation (combustion) was studied. Small substitutions of Pt moles for Pd moles resulted in an increased CH4 oxidation activity in comparison to monometallic Pd. Greater substitutions led to decreased activity. In terms of sulfur poisoning, DRIFTS and TPD studies show that SO2 sorption characteristics depend on both precious metal crystallite particle size and Pd:Pt mole ratio. Catalysts with a small particle size or high Pd content tended to form more aluminum sulfate species, which decomposed at high-temperature. Large particle size or low-Pd content catalysts tended to form more low-temperature decomposing and desorbing species, such as molecular SO2 and aluminum surface sulfite. It was found that the amount of SO2 adsorbed and later desorbed during TPD decreased with increasing particle size or Pt content in the bimetallic Pd-Pt/Al2O3 catalysts. To decouple particle size and mole ratio aspects during CH4 oxidation experiments, catalysts with various metal compositions but similar particle sizes were compared. CO and SO2 adsorption DRIFTS studies were used to identify sites impacted by SO2 exposure and evaluate the Pd:Pt mole ratio effect on sulfur surface species formation. Temperature-programmed oxidation, desorption, and reduction processes were used to evaluate sulfur species decomposition and performance regeneration effectiveness. At low temperatures, Pd-based catalysts with little to no Pt substitution tended to form aluminum sulfate species, which could be removed at high temperatures to recover catalytic activity, but Pd-based catalysts with higher Pt substitution were less effective at sulfate formation at low temperatures. In this case, molecular SO2 and aluminum surface sulfite species inhibited the CH4 oxidation reaction over a broader temperature range. In general, the effectiveness of SO2 regeneration methods decreased with increasing Pt content. For bimetallic catalysts containing more Pt moles than Pd moles, sulfur decays CH4 oxidation activity to a lesser degree than the high-temperature regeneration methods due to the associated sintering effects. In terms of H2O impact, although the CH4 oxidation reaction was inhibited in the presence of H2O for catalysts containing Pd, only low-Pd content catalysts decayed due to H2O exposure.